Tag Archives | sable

Modifier relationships – it’s complicated


In the most recent post in this series on pigment-type switching, I talked about the fact that both horses and dogs have a wild color that is a combination of red and black pigments: bay in horses, and wolf sable in dogs. Those wild colors involve the interaction of two pigment-type switching genes, Extension and Agouti. Horses and dogs also have a mutation for all-red (located at Extension) and another for all-black (located at Agouti).

One of the most straight-forward ways to explain how this works in horses is to say that the two alleles at Extension determine if black hair is possible, and the alleles at Agouti direct the placement of the black pigment, when it is present. This flow chart might help visualize this.


So far the alleles we have here for dogs line up with those for horses. But dogs have a lot more options at both of these loci, and a look at how those work together is a great way to appreciate the complex nature of pigment-type switching, and why the chart above is only part of the picture.


I mentioned that in dogs one of the alleles at Extension, melanistic mask (EM), directs the placement of black pigment. Those familiar with horse color tend to think that is the role of the Agouti locus, which is why it is so tempting to think of Extension as the primary control, and Agouti as the modifier. However, when it comes to the alleles that control the two basic pigments, the loci themselves do not really have simple primary vs. modifier relationships. It is the individual alleles that act as modifiers, and just which allele plays that role varies with the different colors. In fact, as we’ll see later, in some cases there are multiple layers to these relationships.

So the melanistic mask allele (EM) directs the placement of the black pigment, but like horses, dogs have alleles at Agouti that also do this. In addition to the melanistic mask, the dog in the picture above has the Agouti allele for fawn (Ay), which in some breeds is called sable. Another common allele at this locus is black-and-tan (at). That is the color associated with breeds like the Doberman or Rottweiler. Both the fawn and the black-and-tan alleles direct the placement of black pigment into specific patterns.


These two alleles can work simultaneously with the allele for melanistic mask. Fawn dogs with melanistic masks are common in many breeds. This dog has both the black-and-tan pattern and the melanistic mask, which has partially obscured the tan patches that are so clearly visible on the nose of Doberman above. (This was probably even more striking before age caused the muzzle of this dog to turn gray.)


When present, black-and-tan and melanistic mask are both visible in the final coat. There are also alleles at Extension that modify specific alleles at Agouti, changing the nature of the original color. In Salukis, the color known as grizzle is an allele at Extension (EG) that modifies black-and-tan (at) . A separate allele at Extension (eh) alters black-and-tan in a similar fashion to produce “sable” English Cocker Spaniels. (The linked articles are in German, but they contain numerous images of sable (zobel) Cocker Spaniels.)

Saluki photo by Pleple2000, courtesy of Wikimedia Commons.

As the grizzle Salukis and “sable” Cocker Spaniels show, it is possible for Extension to enable the production of black pigment, direct its distribution and even modify the distribution of black pigment set down by alleles at Agouti. It is, in fact, even more complicated than this, because recent studies suggest that the black-and-tan pattern is itself a modification of a pattern known as saddle tan, which is a separate allele at a completely different locus. So grizzle is probably a modification of a modification, with the “original” pattern of red and black pigment residing at a locus that is neither Extension or Agouti.

This shows that even with dogs, where pigment-type switching has been more extensively studied, we only have a partial picture of the process. However, if we place the pieces of the puzzle that were outlined in this post into our previous flow chart, it gives a broader picture than the small window that the (known) alleles in horses allow. Here is an expanded chart showing the relationships between these alleles. If you hover over the image with your mouse, it will drop out the additional alleles to show the more limited picture provided by those alleles that correspond with the pigment-type switches in horses.

Saluki photo by Pleple2000 and Cocker photo by Louis Mayer, courtesy of Wikimedia Commons.

To make things more complicated, even this expanded flow chart covers only a portion of the pigment-type switches in dogs. While horses have two major sites that are known to control pigment-type (Extension and Agouti), dogs have a third (K Locus). That third locus, which controls dominant black and brindle, is an interesting topic for another day, but I want to take us back to horses, and what this bigger (if incomplete) picture might tell us about some of the mysteries still unsolved in that species.

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Merle and black pigment


As I mentioned in a previous post, I was fortunate enough to get some wonderful comparison shots while I was in Kentucky. It seems a good time to share this one since I touched on the topic of merle in dogs in the last few posts. This is Lily, a sable merle Collie. Here she is with her ordinary sable companion.


As I had mentioned in a previous post about merle in dogs, the merle gene effects black pigment. This is why the sabled areas on head are muted on Lily compared to her friend. With only a small amount of black pigment present, the effect is pretty subtle. From a distance Lily looks a little washed out and pale. Her blue eye is quite a giveaway for the merle gene, since those happen even with merles that are completely red pigmented. Her ears, which are often the blackest area on a sable Collie, are the other giveaway.


Her other ear and the colored area along her back skull are all visibly merled.

I should also clarify that she is what Collie breeders call a color-headed white with a single merle gene. She is extensively white because she is homozygous for the color-headed gene, and not because she has two copies of the merle gene.

Here is a typical (non-merle) color-headed Collie. (Photo from Wikimedia.)


Unlike double merles – and the white Boxers in the previous post – the gene responsible for this pattern usually leaves the head, nose and ears dark.

Here is my friend Andrea Caudill’s merle Cocker Spaniel, Domino. (Thank you, Andrea, for letting me use pictures of your sweet boy!)


Domino is probably a double merle. Although many double merles are very white, especially on the face, some carry enough color that they could be mistaken for a dog that had a single merle gene and one of the more extensive piebald patterns.

Since posting about the fact that merle acts on black pigment, several people contacted me about red dogs that were merle. In some breeds, red is used to describe liver, which is a form of black pigment. The people writing were not talking about liver red. In these cases, what was meant was truly red (or yellow) pigment. Some of the dogs did in fact look red, or had what looked like merling in their red-pigmented areas as well as their black. It was often less extensive, but still it was enough to make me wonder how absolute the connection was to black pigment.

If you look closely at Lily (the images link to larger pictures), the area at the corner of her blue eye, running towards the blaze, is roaned. Here is a sable Dachshund with an even more merling in the red areas of his face. (For comparison, here is a sable Dachshund puppy with the merling mostly confined to his sable areas.)

It is also true that merle, when paired with the harlequin (Harl), will effect red pigment. In Danes these dogs are called fawnequins. (Image from Wikimedia.)


The harlequin gene has much the same effect on brindles that inherit the merle gene, which are known as brindlequins. Compare that to a more typical brindle merle here, where the red/yellow pigment is not especially altered.

So it appears that the link between merle and black is not absolute. I know I’ll be looking at merles with red pigment more closely in the future. I am curious to see how common this is, and why it happens in some red dogs but not most.

(I promise to return to horse colors with the next post, which will have the Dominant White stallion Sato!)

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